In typical networking devices, various switches are used in integrated circuits to support different functionalities. For example, a cellular transceiver may support carrier aggregation, which allows for the simultaneous reception of two independent frequency channels. In order to provide simultaneous reception, at least two receive mixers, each driven by an independent frequency divider, are used. These frequency dividers are driven or clocked by one of two different voltage-controlled oscillators (VCOs) that run concurrently. Each divider clock input is also selectable between the two VCOs, and a two-to-one input switch network is often used.
In these types of circuits, the VCOs generally run simultaneously at different frequencies. Therefore, a high degree of isolation between switches is desirable to reduce energy coupling through a disabled switch. For example, when energy passes through a disabled switch, a spur could occur on the input to the frequency dividers. The spur may propagate through the frequency dividers and onto the receive mixer, resulting in unwanted signals mixed into the desired band. By providing isolation between the input switches, energy passing through a disabled switch is reduced.
A multiple switch circuit may be used to provide high isolation. For example, good isolation can be achieved by using two switches in series with a ground shunt switch placed between them. However, this type of scheme requires the two series pass switches to be at least two times larger to provide a comparable resistance of a single switch circuit. Consequently, the total parasitic capacitance of these two series switches in combination with the shunt switch is about four times larger than the capacitance associated with a single switch. This not only limits the operating frequency range of these switches, but also increases power consumption.